Owing to their diverse functionality, high surface-to-volume ratio, and unique
size-dependent properties, nanostructures are of immense importance for chemical
and biological sensors, medical devices, catalysts, photovoltaics, and nanodevices.1-5
A wide range of material choices coupled with different synthetic strategies
result in different morphological versions such as nanometer scale thin films,
nanowires, nanotubes, nanobelts, nanoparticles, and nanoporous structures.2,5,6

Another challenge is to impart multifunctionality to these nanostructures.
Towards this end, heterostructuring of nanostructures by combining two nanostructures
of same or different materials is of significant interest.7
Thus, Professor
Chopra's research combines expertise in nano/microfabrication, nanostructure
growth, materials chemistry, and characterization and spectroscopic techniques
to develop novel nanoscale heterostructures. Professor
Chopra's current focus is on developing graphene encapsulated metal nanoparticles
(a core/shell geometry) and hierarchical heterostructures such as nanowires
coated with nanoparticles.11-15 In a recent
reports, Professor
Chopra has demonstrated some unique growth techniques for fabricating such
heterostructures through very simple and surfactant-free synthetic routes. Such
an approach makes them readily available for their future applications.

Graphene Encapsulated Gold Nanoparticles

By utilizing our CVD design expertise, Professor
Chopra was able to develop CVD methods that have the capability to grow
CNTs, nanowires, or graphene shells. A typical CVD process employs a feed of
chemical precursors undergoing a reaction at high temperature to result in a
precipitated nano, micro, or macro structures.

In order to grow a graphene shell around a metal nanoparticle such as gold
(generally, considered as an inert), our approach utilizes patterned arrays
of gold nanoparticles on a silicon wafer as a catalyst for the growth of graphene
shell in the presence of a hydrocarbon source at temperatures between 600 - 700 °C.11 The grown graphene shells encapsulating
gold nanoparticles were characterized using high resolution electron microscopy
(Figure 1).11

The graphene shell thickness as low as ~1 nm and its growth rate of ~8 nm/h
indicates versatile tunability of our CVD growth method. As compared to previous
approaches used for encapsulating gold nanoparticles inside a graphene shell,
our novel approach demonstrates that CVD method can lead to scalable growth
of such core/shell nanoparticles. Such novel nanoparticles have immense potential
for advanced chemical and biological analysis and medical devices.

Hierarchical Heterostructures

An interesting approach recently reported by us involves a simple surfactant-free
thermal route to develop CuO nanowires coated with Co3O4
nanoparticles.13 Our fundamental studies of their
crystal structure and interfaces showed a unique interfacial relationship between
monoclinic CuO nanowires and spinel Co3O4 nanoparticles,
leading the nanoparticles to uniformly disperse on the nanowires.

Professor Chopra
also performed a comprehensive study pertaining to the morphological evolution
of Co3O4 nanoparticles on CuO nanowires. This part of
the study uniquely considers thermodynamics of the growth process that facilitated
the surface migration of Co3O4 nanoparticles from base
of nanowires along their length (Figure 2).

Such a study relating different phenomena as listed in figure 2 for the development
of hierarchical heterostructures is unique. These kinds of multifunctional and
multicomponent hierarchical heterostructures are extremely useful and will definitely
impact our lives in many ways from automobiles to nanoelectronics. The challenge
is to take them to the next level of innovation, which we are consistently striving
for.